Most conventional Global Navigation Satellite System (GNSS) receivers utilize one or more correlators to acquire, track, and demodulate signals transmitted from orbiting satellites. For example, each Global Positioning System (GPS) satellite transmits a signal with a 1.023 Mchip per second spread rate over a carrier frequency at 1575.42 MHz. Each satellite transmits a unique pseudo-random noise code (PRN) which identifies that particular satellite and facilitates simultaneously receiving signals transmitted by a plurality of satellites in view of the receiver. The PRN code is 1023 chips, corresponding to a 1 millisecond time period. One or more correlators are used to track a PRN signal by comparing the PRN signal received from a satellite with an internally generated reference signal. Typically, the correlation process includes multiplying and integrating the received and internally generated signals. The correlator then generates a correlation signal indicating the result of the comparison. The power of the correlation signal is often accumulated for a period of time known as the “pre-detection interval” (PDI). The accumulated result is then used to adjust the timing of the internally generated signal in order to align it more closely with the received signal in the next pre-detection interval.
FIG. 1A is a block diagram of a conventional GPS correlator 100. Correlator 100 comprises an input 101 in which a GPS baseband signal is input into a multiplier 105 which receives an internally generated PRN signal from a PRN generator 111 via input 110. A resulting set of signals is accumulated and integrated by summing component 120. During acquisition, a comparator 125 indicates when a match between the internally generated PRN signal and the received PRN signal has occurred based upon a pre-set threshold level. A processor 130 receives the signals from comparator 125 and outputs a signal to numerically controlled oscillator (NCO) 135 which controls the timing of the PRN signal from PRN generator 111. As a result of these adjustments, the received signal and the internally generated signal may become aligned in time. Typically, in steady state tracking the signal from summing component 120 is accessed directly by processor 130.
FIG. 1B shows an exemplary correlation function 140 generated by a convention GPS correlator (e.g., 100 of FIG. 1A). In many global positioning systems, this is referred to as a “correlation function.” A typical GPS correlation function is a substantially triangular peak with some rounding and asymmetry introduced due to signal filtering and other effects. A maximum accumulated value 150 is shown at the top of the triangular peak for a given pre-detection interval. Thus, the internally generated signal is punctual (“P”) with respect to the received signal. Using a single correlation function, it is difficult to determine whether to advance or delay the internally generated PRN signal (e.g., from PRN generator 111) such that it is aligned with the received PRN signal in the next pre-detection interval. As a result, some receivers use at least one other correlator which generates an internally generated PRN signal which is leading, or early (“E”), with respect to the received signal. Similarly, the internally generated signal may be lagging, or late (“L”), with respect to the received signal
Under most conditions, when the punctual internally generated PRN signal is aligned with the received PRN signal, the early correlator and late correlator produce accumulated results which have a known relationship with respect to each other and with the accumulated result from the punctual correlator. As shown in FIG. 1B, points 161 and 162 have the same value when the result from the punctual correlator is aligned with the received PRN signal (e.g., point 150). Often, an early-minus-late correlator subtracts the late correlation signal from the early correlation signal. Thus, if the internally generated P signal was early with respect to the received PRN signal, a negative value would be obtained from the early-minus-late correlator. Similarly, if the internally generated P signal was late with respect to the received PRN signal, a positive value would be obtained from the early-minus-late correlator. Ideally, if the P signal is aligned with the received PRN signal, a value of zero would be obtained by the early-minus-late correlator. The term “integrity” refers to correctly identifying and tracking the received PRN signal using the internally generated PRN signal.
The Galileo satellite navigation system also transmits a PRN code in the L1 band which is centered at 1575.420 MHz and has a transmitted bandwidth of 40.92 MHz. For example, a PRN signal with a chipping rate of 1.023 MHz from a Galileo satellite is multiplied by a 1.023 MHz square wave signal to produce a Binary Offset Carrier (BOC (1,1)) signal which is transmitted by a navigation satellite. It is noted that other ratios of the chipping rate and square wave signal may be used. FIG. 2A is a block diagram of a conventional Galileo correlator 200. Correlator 200 comprises an input 201 in which a received BOC (1,1) signal is input into a multiplier 205. Multiplier 205 also receives a BOC (1,1) reference signal from multiplier 250 in which an internally generated PRN signal from a PRN generator 240 is multiplied with a square wave signal 245 which is generated by, for example, a 1.023 MHz clock. A resulting set of signals is accumulated and integrated by summing component 220. During acquisition, a comparator 225 indicates when a match between the internally generated BOC (1,1) reference signal and the received BOC (1,1) signal has occurred based upon a pre-set threshold level. A processor 230 receives the signals from comparator 225 and outputs a signal to numerically controlled oscillator (NCO) 225 which controls the timing of the PRN signal from PRN generator 240. Typically, in steady state tracking the signal from summing component 220 is accessed directly by processor 230.
FIG. 2B shows a correlation function 255 generated by a conventional Galileo correlator (e.g., 200 of FIG. 2A). As shown in FIG. 2B, the correlation function generated when a BOC (1,1) reference signal is combined with a received PRN signal shows a sharp central peak 260 as well as two smaller negative side peaks 261 and 262. The difficulty with using a BOC (1,1) reference signal is that a correlator may latch onto any of peaks 260, 261, or 262 when attempting to align its internally generated BOC (1,1) reference signal with the received BOC (1,1) signal. In other words, rather than correctly aligning the internally generated P signal with peak 260, a receiver may attempt to align the P signal with either of peaks 261 or 262, resulting in an error of approximately 150 meters in determining the geographic position of the receiver. Additionally, the polarity of the correlation function flips 180 degrees based upon what data bit is received. Thus, while correlation function 255 shows a data bit with a value of one, a data bit with a value of zero would show peak 260 pointed downward and peaks 261 and 262 pointed up. As a result, reading the sign of the voltage of correlation function 255 does not facilitate determining whether the internally generated PRN signal is correctly aligned with the received BOC (1,1) signal.